An example of radiographic apparatus is an imaging apparatus that obtains X-ray images by detecting X rays. This apparatus used an image intensifier as an X-ray detecting device in the past. In recent years, a flat panel X-ray detector (hereinafter called simply “FPD”) has come to be used instead.
The FPD has a sensitive film laminated on a substrate, detects radiation incident on the sensitive film, converts the detected radiation into electric charges, and stores the electric charges in capacitors arranged in a two-dimensional array. The electric charges are read by turning on switching elements, and are transmitted as radiation detection signals to an image processor. The image processor obtains an image having pixels based on the radiation detection signals.
The FPD is lightweight and free from complicated detecting distortions compared with the image intensifier used heretofore. Thus, the, FPD has advantages in terms of apparatus construction and image processing.
However, when the FPD is used, the X-ray detection signals include lag-behind parts. A lag-behind part results in an afterimage from X-ray irradiation in a preceding imaging event appearing as an artifact on a next X-ray image.
Particularly, in a fluoroscopy that performs X-ray irradiation continually at short time intervals (e.g. 1/30 second), time lags of the lag-behind parts have influences serious enough to hinder diagnosis.
Artifacts due to lag-behind parts are reduced by reducing long time constant components of the lag-behind parts by using backlight (see Patent Document 1, for example), or by regarding the lag-behind parts as a total of exponential functions having a plurality of time constants, and performing a lag correction by recursive computation using these exponential functions (see Patent Document 2, for example).
Where backlight is used as disclosed in the Patent Document 1 noted above, the construction becomes complicated by a construction required for backlight. Particularly where backlight is used in an FPD having a lightweight construction, the construction must become heavy and complicated again. In the case of Patent Document 2, the lag correction must be carried out by performing recursive computations the number of times X-ray detection signals are sampled. This renders the lag correction complicated and cumbersome.
In order to remove lag-behind parts included in X-ray detection signals simply from the X ray detection signals, it is conceivable in performing a lag correction, to acquire a plurality of X-ray detection signals in time of non-irradiation before irradiation of X rays in an imaging event, acquire a lag image based on the X-ray detection signals, and use this image to remove the lags from a product X-ray image.
On the other hand, apart from the above method of acquiring an X-ray image by lag correction, an energy subtraction (DES) method (hereinafter referred to simply as the “DES method” as appropriate) is known, which acquires a new image from two images, i.e. an X-ray image obtained by high energy X-ray irradiation and an image by low energy X-ray irradiation (see Patent Documents 3-6, for example). This method is used when obtaining chest images in which rib signals are suppressed, for example. As shown in FIG. 11, the DES method performs, for example, a first irradiation for 120 kV and 10 ms, and immediately thereafter performs a second irradiation for 60 kV and 50 to 100 ms. In practice, an idle time of about 200 ms occurs between the first irradiation and second irradiation due to the functional limitations of the apparatus. A subtraction process is carried out by appropriately weighting the images obtained by the first irradiation and second irradiation, respectively. The subtraction process is carried out using the following equation (1), for example:IDES=I1×W1−I2×W2  (1)where, IDES is an energy subtraction image, I1 is an X-ray detection signal (intensity) acquired from the first irradiation, I2 is an X-ray detection signal (intensity) acquired from the second irradiation, W1 is a weight of I1, and W2 is a weight of I2. Instead of being limited to the above equation (1), subtraction may be carried out between respective logarithms as in Patent Documents 3-6 noted above.
[Patent Document 1]
Unexamined Patent Publication No. H9-9153 (pages 3-8, FIG. 1)
[Patent Document 2]
Unexamined Patent Publication No. 2004-242741 (pages 4-11, FIGS. 1 and 3-6)
[Patent Document 3]
Unexamined Patent Publication No. 2002-171444
[Patent Document 4]
Unexamined Patent Publication No. 2002-152594
[Patent Document 5]
Unexamined Patent Publication No. 2002-152593
[Patent Document 6]
Unexamined Patent Publication No. 2000-121579